Thermal and Microwave-Assisted Synthesis of Diels−Alder Adducts of [60]Fullerene with 2,3-Pyrazinoquinodimethanes: Characterization and Electrochemical Properties

[4+2] Cycloaddition reactions of six-membered heterocyclic o -quinodimethanes, generated “in situ” from pyrazine derivatives, to [60]fullerene, either under thermal or microwave irradiation are described. Other microwave assisted cycloadditions involving o -quinodimethanes derived from thiophene were also performed. A comparative study of the activation energy for the boat-to-boat conformational inversion has been carried out by dynamic NMR experiments, the Δ G ⧧ values being highly dependent on the nature of the covalently attached heterocyclic systems. Theoretical calculations predict a more planar cyclohexene ring for the five member containing cycloadducts. The cycloaddition process is controlled by the HOMO of the heterocyclic o -quinodimethanes showing a LUMO(C 60 )-HOMO(diene) energy differences typical for favoured cycloadditions. The redox properties of the novel organofullerenes have been determined by cyclic voltammetry in solution, showing a cathodically shifted first reduction potential values, related to [60]fullerene. Compound 15c bearing two cyano groups exhibited an opposite trend which was accounted for by the lower LUMO energy determined by semiempirical calculations.